Capstone: Keypress → Pixel

The finale of the track: one keypress, traced through every rung this track built, from the switch under the key to the pixel it finally lights.

Before you start. The lesson on I/O, interrupts and the bus gave you the last piece: a device raises a flag instead of waiting to be asked, and a DMA controller moves data around without the CPU touching it. This lesson adds no new piece; it sends one keypress up every rung you already have, in order, and times the whole trip.

The idea in 30 seconds

A keypress becomes a number before it becomes a letter: the keyboard sends a scancode, and only a convention turns one byte into ‘A’.
The keyboard interrupts the CPU rather than waiting to be asked, which is why the CPU can do other work between keystrokes instead of checking the keyboard in a loop.
Waking your program and running it again is a context switch: the scheduler puts it back exactly where it left off, and your program never notices the gap.
That same byte then passes through a register, a cache, RAM, and your own program’s stack and heap before it becomes a shape.
The letter’s shape is drawn into a patch of memory that mirrors the screen, and the graphics chip copies it to the display without the main processor touching it again.
Every layer in this chain hides the one below it, which is the one habit that makes a machine this complicated buildable, and the one habit that, once you can see through it, lets you find where anything actually broke.

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